There is a need to minimise failures within the built environment. In recent years, this has been driven not only by safety considerations, but also by a need to optimise the use of limited natural resources.
As such, it is necessary to monitor built structures, in order to identify potential failures. A problem arises when the built structure is inaccessible, e.g. if the structure is at a subsurface location.
Subsurface concrete structures are generally made by pouring fresh concrete into a cavity (such as an underground borehole, or a shuttered structure to contain the concrete). They require non-visual assessment after curing to ensure that the poured concrete is present in the designed quantity at all points in the structure and that there are no voids, bulges or significant anomalies in the concrete.
In some cases, strain gauges are used to facilitate non-visual assessment of subsurface structures after curing. The non-visual assessment of subsurface structures after curing may also be done using a technique known as cross-hole sonic logging.
However, in the case of subsurface structures made from concrete poured in situ, a potential failure may arise during the construction of the structure. There is therefore a need to identify potential failures as early as possible, particularly in those subsurface structures that are used to support or retain other structures, such as concrete piles or diaphragm walls.